1. Field of the Invention
The present invention relates to a power unit including an engine, a belt transmission driven by the engine, and a second transmission, such as a gear transmission, driven by the belt transmission, and relates to a vehicle, especially, a multi-wheel drive vehicle such as a four-wheel drive vehicle, equipped with the power unit.
2. Related Art
As disclosed in JP 2006-76548 A, a four-wheel drive utility vehicle, serving as a conventional typical multi-wheel drive vehicle, is equipped with a power unit for driving both a front wheel axle and a rear wheel axle. The power unit includes an engine, a belt transmission driven by the engine, and a second transmission, such as a gear transmission, driven by the belt transmission. The second transmission includes a first output portion for driving one of the front and rear wheel axles, and a second output portion for driving the other of the front and rear wheel axles.
This conventional power unit is configured so that, when it is equipped on a vehicle, an engine output shaft of the engine is extended to have a horizontal axis in a lateral direction of the vehicle, and the second transmission is disposed immediately rearward (or, when occasion demands, immediately forward) of the engine so as to extend its own transmission shafts having horizontal axes in the lateral direction of the vehicle. One of the transmission shafts is a transmission input shaft serving as an input shaft of the second transmission. The engine output shaft and the transmission input shaft project, at respective end portions, thereof outward from the engine and the transmission toward one of either the right or left sides of the vehicle. The belt transmission is disposed on either a right or left side of the engine and the second transmission so as to drivingly connect the outwardly projecting end portion of the transmission input shaft to the outwardly projecting
end portion of the engine output shaft. Hereinafter, this conventional power unit for a multi-wheel drive vehicle is referred to as a lateral axial power unit.
In the lateral axial power unit as disclosed in the above-mentioned reference, the second transmission carries and drives a rear wheel axle so as to serves as a rear transaxle of the vehicle. In the vehicle, a front transaxle carrying and driving a front wheel axle is disposed opposite to the second transmission in the fore-and-aft direction of the vehicle with respect to the engine. The second transmission has a gear housing portion for driving the front transaxle, serving as the above-mentioned second output portion for driving the front wheel axle. The gear housing portion projects outward from the second transmission laterally opposite to the belt transmission, and a front-wheel driving power take-off (PTO) shaft projects forward from the gear housing portion, whereby the gear housing portion, the PTO shaft, and a propeller shaft extended from the PTO shaft toward the front transaxle are prevented from interfering with the engine and the belt transmission.
In the lateral axial power unit, the second output portion of the second transmission can overlap the belt transmission in the vertical direction of the vehicle so as to reduce a vertical length of the power unit because it is disposed opposite to the belt transmission in the lateral direction of the vehicle. However, the lateral axial power unit is restricted in its minimization in the lateral direction of the vehicle, because of the horizontal axes of the engine output shaft and the transmission shafts of the second transmission extended in the lateral direction of the vehicle, and because of the lateral opposite arrangement of the belt transmission and the second output portion of the second transmission.
As disclosed in each of JP H02-200526A, JP H10-297295A and U.S. Pat. No. 7,377,351, an alternative conventional power unit for a multi-wheel drive vehicle includes an engine, a belt transmission driven by the engine, and a second transmission having first and second output portions for respective front and rear wheel axles and driven by the belt transmission. In this power unit, an engine output shaft of the engine has a horizontal axis in a fore-and-aft direction of a vehicle. The second transmission is disposed on one of right and left sides of the engine. Transmission shafts of the second transmission have respective horizontal axes in the fore-and-aft direction of the vehicle.
One of the transmission shafts of the second transmission is a transmission input shaft serving as an input shaft of the transmission shafts. The engine output shaft and the transmission input shaft have end portions projecting forward or rearward from the engine and the second transmission in the vehicle. The belt transmission is disposed on one of the front and rear sides of the engine and the second transmission so as to drivingly connect the end portions of the engine output shaft and the transmission input shaft to each other. More specifically, when the belt transmission is disposed on the front or rear side of the engine and the second transmission, a drive pulley is provided on the end portion of the engine output shaft, a driven pulley is provided on the end portion of the transmission input shaft, and a belt is looped over the drive and driven pulleys. Hereinafter, this power unit for a multi-wheel drive vehicle is referred to as a fore-and-aft axial power unit.
As understood from the above description, in comparison with the lateral axial power unit, the fore-and-aft axial power unit is advantageous in its minimization in the lateral direction of the vehicle because the engine output shaft and the transmission shafts of the second transmission have respective horizontal axes in the fore-and-aft direction of the vehicle, and the belt transmission is disposed on one of front and rear sides of the vehicle.
In each of the fore-and-aft axial power units disclosed in the above-mentioned three references, the second transmission includes a transmission output shaft for driving both a front wheel axle and a rear wheel axle. In this regard, a front transaxle carrying and driving the front wheel axle is disposed forward from the power unit, and a rear transaxle carrying and driving the rear wheel axle is disposed rearward from the power unit. A front end portion of the transmission output shaft projects forwardly outward from the second transmission so as to be drivingly connected to the front transaxle, and a rear end portion of the transmission output shaft projects rearwardly outward from the second transmission so as to be drivingly connected to the rear transaxle. Thus, one of the front and rear end portions of the transmission output shaft projects outward from the second transmission at the front or rear side of the second transmission that is on the same side as the belt transmission. This front or rear end portion of the transmission output shaft is disposed below the end portion of the transmission input shaft, and more specifically, below a bottom end of a portion of the belt transmission having a portion with the belt wound around the driven pulley on the end portion of the transmission input shaft. Hereinafter, this portion of the belt transmission is referred to as the portion of the belt transmission around the driven pulley, or is simply referred to as the portion of the belt transmission. In other words, at the front or rear side of the second transmission, the end portion of the transmission output shaft and the portion of the belt transmission around the driven pulley are vertically offset from each other, thereby being prevented from interfering with each other.
Regarding heights of the respective end portions of the engine output shaft and the transmission input shaft, as disclosed in U.S. Pat. No. 7,377,351, the end portion of the engine output shaft is lower than the end portion of the transmission input shaft so that a portion of the belt wound around the drive pulley is lower than the portion of the belt wound around the driven pulley, thereby vertically slanting the belt transmission. Alternatively, as disclosed in JP H02-200526A, the end portion of the engine output shaft and the end portion of the transmission input shaft are leveled at substantially the same heights so that the belt has a substantially horizontal upper or lower portion between the drive and driven pulleys.
JP H02-200526A also discloses an alternative fore-and-aft axial power unit. In this power unit, at either the front or rear end of the second transmission from which the end portion of the transmission input shaft projects outward, the end portion of the transmission output shaft projects forward or rearward on a lateral distal side of the end portion of the transmission input shaft (i.e., laterally opposite to the end portion of the engine output shaft with respect to the end portion of the transmission input shaft), and more specifically, on a lateral distal side of a lateral distal end of the portion of the belt wound around the driven pulley (i.e., laterally opposite to the drive pulley with respect to the driven pulley). In other words, the end portion of the transmission output shaft is offset from the belt of the belt transmission in the lateral direction of the vehicle.